3, 4-dicarboloweralkoxy-1, 2, 5-thiadiazoles and process



United States Patent 6 Claims. (Cl. 260-302) This invention relates to new organic compounds. More specifically it relates to new heterocyclic compounds wherein the hetero atoms are nitrogen and sulfur. Still more specifically, it is concerned with 1,2,5- thiadiazoles substituted at the 3 and 4 positions with carboxy groups, and with derivatives thereof.

This application is a division of our copending application Serial No. 750,419, filed July 23, 1958, now Patent No. 2,980,687.

Many heterocyclicorganic compounds are known in the scientific literature and have been known for many years. However, prior to this invention, 3,4-disubstituted 1,2,5-thiadiazoles were completely unknown. It is an object of this invention to provide for the first time 1,2,5-thiadiazoles substituted at the 3 and 4 positions of the heterocyclic ring. It is a further object to provide a synthesis of such compounds from 4-nitro-2,1,3- benzothiadiazole. A still further object is the provision of 1,2,5-thiadiazole-3,4-dicarboxylic acid by oxidation of 4-nitro-2,1,3-benzothiadiazole. An additional object is the provision of mono and disalts of 1,2,5-thiadiazole- 3,4-dicarboxylic acid, and of the hydrocarbon esters of said acid. Still another object is the provision of the amides and acid halides derived from the aforementioned dicarboxylic acid. It is yet another object to make available syntheses of such compounds.

1,2,5-thiadiazole-3,4-dicarboxylic acid is a new compound having the following structural formula:

N COOH /a St N COOH This compound is strongly acidic and very soluble in water and polar organic solvents. When pure it exists as a high melting crystalline solid.

It has been discovered that 1,2,5-thiadiazole-3,4-dicarboxylic acid may be prepared by oxidation of 4-nitro- 2,1,3-benzothiadiazole with permanganate under the con ditions described below. This process may be pictured structurally as follows:

N02 '1 1: N\ coon 5 S2MXIOFS\ i s 7 \N/ rz ooon and 75 C., and preferably in the range of 50-70 C. For best results from 68 moles of permanganate are employed for every mole of 4-nitro-2,1,3-benzothiadiazole. Six moles are required by the stoichiometry of the reaction and only a slight excess over this theoretical amount has been necessary to obtain satisfactory results. Larger quantities of oxidizing agent may, of course, be employed if desired.

For convenience in conducting the reaction it is preferred to add the permanganate salt to a solution of 4- nitro-2,1,3-benzothiadiazole gradually over a period of one-half to two hours, either in solution or as a solid. The oxidation proceeds rapidly and is ordinarily substantially complete as soon as the addition of oxidizing agent is finished. As will be recognized by those skilled in this art, completion of the reaction may be readily determined by the persistence in the reaction mixture of the characteristic blue-purple permanganate color.

The pH of the reaction medium is important to the successful oxidation of 4.-nitro:2,1,3-benzothiadiazole to l,2,5-thiadiazole-3,4-dicarboxylic acid- For best results, the initial pH of the benzothiadiazole solution should be about 6-7. As the oxidation proceeds the pH will gradually become alkaline until the final pH is about 10. As long as the pH is about neutral or alkaline the desired product is obtained. An acidic permanganate oxidation, however, should be avoided in the process, i.e. during the oxidation the reaction mixture should not become strongly acidic.

It will be realized that l,2,5-thiadiazole-3,4-dicarboxylic acid is formed in the reaction mixture as a salt,

the particular salt depending upon the permanganate salt used as the oxidant. Thus, when potassium perman ganate is utilized as the oxidizing agent, the dipotassium salt of 1,2,5-thiadiazole-3,4-dicarboxylic acid is formed in the nuetral or alkaline oxidation medium; correspondingly, the sodium salt of the diacid is produced when sodium permanganate is the oxidizing agent. The free acid and/or other salts may be prepared therefrom as discussed below.

1,2,5-tl1iadiazole-3,4-dicarboxylic acid and its alkali metal salts are highly water-soluble substances. For this reason we prefer to purify and isolate the acid by way of a water insoluble heavy metal salt, preferably the mono-silver salt. For example, the aqueous solution obtained upon oxidation of 4-nitro-2,1,3-benzothiadiazole may be filtered to remove insolubles and the resulting filtrate, which contains a water soluble alkali metal salt of 1,2,5-thiadiazole-3,4-dicarboxylic acid, made strongly acidic with nitric acid. A water soluble silver salt such as silver nitrate is then added whereupon the highly in soluble mono-silver salt of 1,2,5-thiadiazole-3,4-dicarboxylic acid precipitates from solution in a highly pure form. This latter salt may then be converted to substantially pure free acid by removal of the silver therefrom with an acid such as hydrochloric, hydrobromic, hydriodic acids or hydrogen sulfide.

According to an additional embodiment of this invention, mono and di salts of l,2,5-thiadiazole-3,4-dicarboxylic acid may be prepared by treating the free acid with a base. In this manner, the ammonium salts, alkali metal salts such as sodium and potassium derivatives, alkaline earth metal salts such as the barium and calcium salts, or heavy metal salts may be obtained. In order to prepare a di-salt an aqueous solution of the free acid is treated with excess base. The mono salts are conveniently prepared by treating an aqueous solution of 1,2,5-thiadiazole-3,4-dicarboxylic acid with a base toa pH of about 3. In synthesizing these salts, 'we prefer. to utilize bases such as ammoniurn hydroxide,'potassium hy droxide, sodium hydroxide and the-like. The salts; areconveniently isolated by precipitation from the aqueous reaction mixture with acetone. Such salts may be further reacted as discussed below to prepare other useful derivatives of 1,2,5-thiadiazole-3,4-dicarboxylic acid.

A still further embodiment of the invention comprises the esters of l,2,5-thiadiazole-3,4-dicarboxylic acid, particularly the alkyl esters. The di-lower alkyl esters are obtained by reaction of an alkali metal or ammonium salt of 1,2,5-thiadiazole-3,4-dicarboxylic acid with an alkanol in the presence of a strong acid. It is desirable to carry out the esterification process in the cold, i.e. at'temperatures of about to 15 C. although higher reaction temperatures may be used if desired. Thus, lower alkanolic esters such as 3,4-dicarbomethoxy-1,2,5-thiadiazole; 3,4- dicarbethoxy-1,2,5-thiadiazole; 3,4-dicarbopropoxy-1,2,5- thiadiazole and 3,4-dicarbobutoxy-1,2,5-thiadiazole are obtained by suspending an ammonium or alkali metal salt of the free acid in the appropriate lower alkanol saturated with dry hydrogen chloride. The reaction mixture is held in the cold for from 20-50 hours in order to obtain maximum esterification. These lower alkyl esters, which are high-boiling oils may be recovered from the reaction mixture by methods known to one skilled in this art. We prefer to employ the mono-ammonium salt of 1,2,5-thiadiazole-3,4-dicarboxylic acid as starting material for the synthesis of the esters because that salt is highly crystal line and readily purified. However, other salts such as the mono-potassium, the monosodium, the diammonium and di-potassium salts could also be utilized for this purpose.

Alternatively, the esters may be synthesized in high yield from 1,2,5-thiadiazole-3,4-dicarboxylic acid itself by treating said acid with an alcohol and the like in the presence of a strong acid.

In a still further embodiment of the invention, the mono esters of 1,2,5-thiadiazole-3,4-dicarboxylic acid may be prepared by treating the mono-silver salt with an alkyl iodide, such as methyl or ethyl iodide, in an organic solvent. Benzene, toluene or ether are particularly suitable solvents for this purpose. In this fashion there may be obtained esters such as the mono-methyl, mono-ethyl, mono-propyl and mono-butyl esters of 1,2,5-thiadiazole- 3,4-dicarboxylic acid.

In accordance with a still further embodiment of this invention there are provided the amide and acid halide derivatives of l,2,5-thiadiazole-3,4-dicarboxylic acid. The amides are synthesized by treating an ester, and preferably a lower alkyl ester, of the free acid with ammonia. For example, 3-carboxamido l,2,5-thiadiazole-4-carboxylic acid is obtained by reaction of a mono lower alkyl ester of 1,2,5-thiadiazole-3,4-dicarboxylic acid with aqueous ammonia. The 3,4-diamide is produced on treatment of lower alkyl 1,2,5-thiadiazole-3,4-dicarboxylate with anhydrous ammonia in the cold.

Alternatively, the diamide may be prepared from the di-acid halide, e.g. the di-acid chloride or bromide, by contacting said material with ammonia. The acid halides are produced on treatment of an alkli metal salt of 1,2,5 thiadiazole-3,4-dicarboxylic acid, such as the potassium salt, with a halogenating agent. Halogenating agents such as thionyl chloride or thionyl bromide are particularly suitable for this purpose.

l,2,5-thiadiazole-3,4-dicarboxylic acid is a useful resin outing agent, being particularly useful in the curing of epoxy resins employed for casting, fiber glass lamination and adhesives.

Also certain of the new compounds of this invention, and particularly 1,2,5-thiadiazole-3,4-dicarboxamide, have significant anti-diabetic activity and may be employed as anti-diabetic agents.

The 1,2,5-thiadiazole-3,4-dicarboxylic acid and the several salts, esters, amides, acid halides and like derivatives described herein may be converted one to another as described in more detail in the detailed examples which follow, such inter-conversions and reactions, and

the new compounds produced thereby are a part of the instant invention.

EXAMPLE 1 1 ,2,5-thiadiaz0le-3,4-dicarboxylic acid 25 grams of 4-nitro2,1,3-benzothiadiazole (0.138 mole) was slurried in 500 ml. of water. The mixture was heated to 60 C. on a steam bath and a solution'of 131 grams of potassium permanganate (0.828 mole) in 2600 ml. of water was added there-to over a period of 45 minutes, the temperature being maintained between 65-70 C. Ex-. cess permanganate remaining at the end of the addition period was destroyed by the addition of a small amount of ethanol. The manganese dioxide was separated from the hot solution by filtration and washed with 400 ml. of water. The filtrate and washings were combined and the clear, light-yellow solution of the dipotassium salt of 1,2,5-thiadiazole-3,4-dicarboxylic acid was acidified to a pH of about 1 by the addition of ml. of concentrated nitric acid. The acidified solution was treated with a so-. lution of 23.5 grams of silver nitrate in 50 ml. of water and allowed to cool in the ice box. The white, crystallized mono-silver salt of 1,2,5-thiadiazole-3,4-dicarboxylic acid was separated by filtration and washed with 200 ml. of water in small portions. The dried silver salt weighed 30 grams and blackened without melting between 235-2S5 C.

The silver salt wasslurried in 300 ml. of water and hydrogen sulfide gas passed into the mixture until the formation of silver sulfide was complete. The resulting suspension was treated with 5 grams of activated charcoal and filtered by gravity; the precipitated silver sulfide was washed with ml. of warm water. The combined filtrates and washes were evaporated to a moist residue and the last traces of water were removed by drying over phosphorus pentoxide for a few hours. The 1,2,5-thiadiazole-3,4-dicarboxylic acid thus obtained weighed 14.3 grams. The product melted at l74178 C. with decomposition. Recrystallization from glacial acetic acid raised the melting point to 184 C.

Analysis.Calculated for C H N SO C, 27.59; H, 1.15; N, 16.09; S, 18.41. Found: C, 28.09; H, 1.26; N, 16.19; S, 18.1.6.

On standing the substance absorbs water and is converted to the monohydrate.

Analysis.-Calculated for C H N SO C, 25.00; H, 2.10. Found: C, 25.04; H, 2.08.

EXAMPLE 2 Salts of 1,2,5-tiziadiazole-3,4-dicarboxylic acid A. Mono-silver salt.Addition of a silver nitrate solution to an aqueous solution of l,2,5-thiadiazole-3,4-dicarboxylic acid precipitated the mono-silver salt of the acid. The product was purified by recrystallization from hot water. It does not melt, but blackens gradually between 235-255 C.

B. Mono-ammonium salt.-Ammonium hydroxide was added to an aqueous solution'of 1,2,5-thiadiazole-3,4-dicarboxylic acid to a pH of 3. On cooling,.some of the salt crystallizes and the remainder is precipitated by the addition of acetone to the filtrate. On recrystallization from water-acetone the mono-ammonium salt is obtained, melting point 253-254 C.

AnalysisCalculated for C H N O S: C, 25.13; H, 2.64; N, 21.99; 5, 16.77. Found: C, 25.65; H, 2.67; N, 21.89; S, 16.53.

C. Diammonium sabr -An excess of ammonium hydroxide was added to an aqueous solution of 1,2,5-thiadiazole-3,4-dicarboxylic acid and an equal volume of acetone added to the ammoniacal solution. The precipitated diamrnonium salt was collected by filtration and dried, The compound was purified by recrystallization from water-acetone, melting point 254-255 C.-

Analysis..-Ca.lculated for C H N SO C, 21.24; H, v

' moved by filtration.

4.66; N, 24.77; S, 14.08. Found: C, 21.66; H, 4.66; N, 24.87; S, 14.01.

D. Mono-potassium salt.Potassium hydroxide was added to an aqueous solution of 1,2,5-thiadiazole-3,4-dicarboxylic acid to a pH of 3 and an equal portion of acetone was then added. The mono-potassium salt crystallized, and salt was purified by recrystallization from water-acetone, melting point 293 C.

Analysis.Calculated for C HN SO K: C, 22.64; H, 0.48; N, 13.20. Found: C, 22.47; H, 0.31; N, 13.21.

E. Dipotassium salt.The dipotassium salt was prepared and purified in the same manner as the diammonium salt using potassium hydroxide in place of ammonium hydroxide. Melting point 310 C.

These salts are useful in purifying the free acid, and as intermediates in the synthesis of esters and acid halides of l,2,5-thiadiazole-3,4-dicarboxylic acid.

EXAMPLE 3 1,2,5-thiadiazole-3,4-dicarboxylic acid di-acid chloride One gram of mono-potassium-1,2,5-thiadiazole-3,4-dicarboxylate was added to 5 ml. of thionyl chloride. The mixture was boiled under refiux for one hour and the excess solvent was then removed in vacuo. The resulting residue was sublimed at reduced pressure (50/3 mm.) and 475 mg. of the white crystalline l,2,5-thiadiazole-3,4- dicarboxylic acid di-acid chloride was obtained. The melting point was 47 C.

Analysis.-Calculated for C Cl N SO Cl, 33.60; S, 15.19. Found: Cl, 33.52; S, 14.70.

I On treatment with ammonia in the cold, this material is converted to 3,4-dicarboxamido-1,2,5-thiadiazole.

. EXAMPLE 4 4-carb0meth0xy-1,2,5-thiadiaz0le-3-carb0xylic acid Two grams of recrystallized mono-silver-l,2,5-thiadiazole-3,4-dicarboxylate was mixed with 20 ml. of dry benzene and 2.3 grams of distilled methyl iodide, and the resulting mixture stirred for 36 hours. The yellow precipitate of silver iodide was removed by filtration and washed well with ether. The combined filtrate and washes were evaporated under reduced pressure, leaving 4-carbo methoxy-l,2,5-thiadiazole-3-carboxylic acid as a clear light yellow oil which slowly crystallized on standing at C. The solid melted at 74-78 C. Sublimation of this material at 60/ 0.1 mm. and then recrystallization of the sublimate from a mixture of ether and petroleum ether raised the melting point to 8081 C.

Analysis.Calcd. for C H N SO C, 31.91; H, 2.14; N, 14.89; S, 17.04. Found: C, 31.70; H, 2.12; N, 14.91; S, 16.77.

EXAMPLE 5 4-carb0meth0xy-1 ,2,5-thiadiaz0le-3-carb0xylic acid A suspension of 8 grams of the mono-silver salt of 1,2, 5-thiadiazole-3,4-dicarboxylic acid in a solution of 6 ml. of methyl iodide in 80 ml. of dry ethyl ether was stirred at room temperature in the dark for 43 hours. The resulting solid was removed byfiltration and treated with a fresh solution of 5 ml. of methyl iodide in ethyl ether for 76 hours. The yellow precipitate of silver iodide was removed by filtration, and the ether solutions combined. Removal of the ether by distillation in vacuo gave 5 grams of 4 carbomethoxy 1,2,5 thiadiazole-carboxylic acid as a pale yellow viscous oil which slowly solidified on standing.

EXAMPLE 6 4-carbaxamido-1 ,2,5-thiadiazole-3-carb0xylic acid 1.27 grams of 4-carbomethoxy-1,2,5-thiadiazole-3-carboxylic acid was shaken vigorously with a solution of 2 ml. of concentrated ammonium hydroxide in 5 m1. of water. A light brown solid separated which was re- This was crude 1,2,5-thiadiazole 3,4-dicarboxamide. The filtrate was acidified with nitric acid, and the light tan solid which formed was collected by filtration. Recrystallization of this material from 50 ml. of acetone gave 4-carboxamide-1,2,5-thiadiazole-3- carboxylic acid as short colorless prisms, melting point 21922-0.5 C.

Analysis.Calculated for C H N O S: C; 27.75; H, 1.75; N, 24.28; S, 18.53. Found: C, 27.88; H, 2.00; N, 24.15; S, 18.32.

EXAMPLE 7 4-carb0meth0xy-1,2,5-thiadiazole-3-carb0nyl chloride 3,4-dicarbomethoxy-l,2,5-thiadiazole An ice-cooled suspension of 2.12 grams of 1,2,5-thiadiazole-3,4dicarboxylic acid mono-ammonium salt in 25 ml. of dry methanol was saturated with hydrogen chloride gas. The reaction mixture was allowed to stand in the refrigerator for 40 hours, filtered, and the colorless inorganic precipitate washed with methanol. The filtrate was concentrated until the formation of two liquid phases. One phase was removed'by extraction of the concentrate with three portions of ether totaling ml. Evaporation of the ether extracts yielded a yellow oil which was distilled at 129/4 mm. pressure to give 3,4-dicarbomethoxy-1,2,5-thiadiazole as a white viscous liquid which soldified in the cold. x max. 263264 log e=4.00 A min. 233 log 6:3.13.

EXAMPLE 9 3,4 -dicarb0meth0xy-I ,2,5 -thiadiazole 3.9 grams of the mono-ammonium salt of 1,2,5-thiadiazo-le-3,4-dicarboxylic acid was suspended in 45 ml. of methanol. The mixture was cooled in ice and saturated with dry hydrogen chloride. It was then held at about 0-5 C. for 46 hours at the end of which time the hydrogen chloride was neutralized by the addition of solid sodium bicarbonate. The resulting white precipitate was removed by filtration and washed with methanol. The combined filtrate and washes were evaporated under vacuum to a yellow oil. Ethyl ether was added to dissolve the oil, and the mixture was decanted from a small water layer. The ether was removed in vacuo, and the oil was distilled at 0.2 mm. Three fractions were obtained over a range of 89-97 C., totaling 3.44 grams. These were allowed to solidify partially at room temperature. Pure 3,4-dicarbomethoxy-1,2,5-thiadiazole was prepared by combining the solid portions of the three fractions and redistilling at 0.1 mm. The product was a colorless viscous liquid, boiling point 84-85 C./0.1 mm.

AnaIysis.--Calcd. for C H N O S: C, 35.64; H, 2.99; N, 13.85; S, 15.88. Found: C, 36.01; H, 3.03; S, 15.77.

EXAMPLE 10 1,2,5-thiadiaz0le-3,4-dicarb0xamide Anhydrous ammonia gas was passed through a solution of 0.75 gram of 3,4-dicarbomethoxy-1,2,5-thiadiazole in 10 ml. of methanol until the solution was saturated. The reaction mixture was then allowed to cool in the cold for several hours. The colorless crystals which deposited were filtered and washed several times with methanol. Recrystallization of the crystals from hot water yielded substantially pure 1,2,5-thiadiazole-3,4-dicarboxamide, melting point 240 C.

Analysis.-Calcd. for C H N O S: C, 27.90; H, 2.34; N, 32.54; S, 18.63. Found: C, 27.95; H, 2.34; N, 32.40; S, 17.52.

EXAMPLE 11 1 ,2,5-thiadiazole-3,4-dicarbxamide 0.504 gram of 4-carbomethoxy-1,2,5-thiadiazole-3-carbonyl chloride was added dropwise to 5 ml. of ice-cold concentrated ammonium hydroxide. 1,2,5-thiadiazo1e-3, 4-dicarboxamide separated as a white, finely crystalline. It was recovered by filtration, washed with methanol and air-dried.

EXAMPLE 12 3,4 -d icarbomethoxy-l ,2,5-thiadiaz0le One gram of 1,2,5-thiadiazole-3,4-dicarboxylic acid was added to ml. of methanol. The resulting mixture was cooled and then saturated with dry hydrogen chloride gas. The mixture was maintained at 0-10 C. for about 30 hours and then the excess acid neutralized with sodium bicarbonate. The resulting precipitate was removed by filtering and the filtrate concentrated to an oil in vacuo. The oily dimethyl ester of 1,2,5-thiadiazole-3,4-dicarboxylic acid was extracted in a small volume of ethyl ether and the ether was then removed by concentration and the oil distilled at a pressure of 0.2 mm. The material boiling in the temperature range of 90-97 C. was collected and further redistilled as described in Example 9 to give substantially pure 3,4-dicarbomethoxy-1,2,5-thiadiazole.

When this process is carried out employing ethanol as the solvent in place of methanol, there is obtained 3,4-dicarboethoxy-1,2,5-thiadiazole.

These esters, and other lower alkyl esters which are prepared from the free acid in the same manner, are useful intermediates in the synthesis of the diarnide as set forth in Example 10.

EXAMPLE 13 3,4-dicyan0-1 ,2,5-thiadiazole A solution of 0.96 gram of 3,4-dicarboxamido-l,2,5- thiadiazole in 6 ml. of phosphorus oxychloride was refluxed for about 40 minutes. The mixture was then chilled and poured onto 250 ml. of crushed ice. The mixture was stirred until the ice had melted and then extracted with three 50 ml. portions of ether. The ether extracts wherein R and R are lower alkyl groups. 2. 3,4-dicarboethoxy-1,2,5-thiadiazole. 3. 3,4-dicarbomethoxy-l,2,5-thiadiazole.

4. The process of preparing a compound of the formula N COOR wherein R is a lower alkyl group which comprises treating a salt of 1,2,5-thiadiazole-3,4-dicarboxylic acid with a lower alkanol in the presence of a mineral acid.

5. The process of claim 4 wherein the acid is a hydrohalic acid.

6. The process of preparing a compound of the formula N COOR S NJKCOOR wherein R is a lower alkyl group which comprises treating 1,2.5-thiadiazole-3,4-dicarboxylic acid with a lower alkanol in the presence of a mineral acid.

References Cited in the file of this patent Khaletskii et al.: Chem. Abstracts, vol. 52, col. 4605-6 (1958). 

1. A COMPOUND HAVING THE FORMULA 